Touchscreen displays are able to detect a touch such as by a finger or stylus within an active or display area. Use of a touchscreen as part of a display enables a user to interact with an electronic application by touching the touchscreen. The display may present images to the user. Such images may include user interface constructs such as different buttons, images, or other regions that can be selected, manipulated, or actuated by touch. Touchscreens can therefore provide an effective user interface for cell phones, GPS devices, personal digital assistants (PDAs), computers, ATM machines, appliances and other devices.
Touchscreens use various technologies to sense touch from a finger or stylus, such as resistive, capacitive, infrared, and acoustic sensors. Capacitive touchscreens often use one or more layers of transverse electrodes, drive electrodes and sense electrodes. In one type of capacitive sensor based touchscreen, a touch changes a capacitance at a node in an array of electrodes overlaying the display device. A node is typically thought of as the area where a drive electrode and a receive electrode overlap or otherwise run adjacent.
Transparent electrodes such as indium tin oxide (ITO) or transparent conductive polymers may be used to form the electrodes. Some layouts of electrodes utilize a flooded type pattern of drive electrodes to shield the sense electrodes from electric field interference from an underlying display such as a liquid crystal display (LCD). The flooded type pattern may use solid fill drive electrode patterns formed in a layer between the sense electrodes and display.
In some prior touchscreen devices, the layer of electrodes closest to the display, are the drive electrodes, and run in a first direction. The sense electrodes include spines that run transverse to the drive electrodes, and may also include crossbars that run in the same direction as the drive electrodes. In such prior devices, the resistance to connection lines from portions of the crossbar electrode farthest from connection lines on the touchscreen is greater than the resistance to connection lines from portions of the crossbars closer to the connection lines. The electronics may contain sense circuitry having a sense capacitor. The sense circuitry operate to accommodate a worst case charge transfer time, having resistive and capacitive components, to allow sufficient charge to transfer from the drive electrodes through the sense electrodes to the sense capacitor. Long charge transfer times could significantly slow down the operation of the touchscreen.